optin_builtin_traits is now called auto_traits

src/4_pin.md

@@ -611,7 +611,7 @@ But now, let's prevent this problem using `Pin`. I've commented along the way to
 make it easier to spot and understand the changes we need to make.
 
 ```rust
-#![feature(optin_builtin_traits, negative_impls)] // needed to implement `!Unpin`
+#![feature(auto_traits, negative_impls)] // needed to implement `!Unpin`
 use std::pin::Pin;
 
 pub fn main() {